Naser A. Al-Shayea

Effects of confining pressure and temperature on mixed-mode (I–II) fracture toughness of a limestone rock

Studying fracture toughness behavior at elevated temperatures and confining pressures is valuable for a number of practical situations such as hydraulic fracturing used to enhance oil and gas recovery from a reservoir, and the disposal or safe storage of radioactive waste in underground cavities. Mixed-mode (I–II) fracture toughness under simulated reservoir conditions of high temperature and confining pressure was studied using straight notched Brazilian disk (SNBD) specimens under diametrical compression. Rock samples were collected from a limestone formation outcropping in the Central Province of Saudi Arabia. Tests were conducted under an effective confining pressure (σ3) of up to 28 MPa (4000 psi), and a temperature of up to 116°C. The results show a substantial increase in fracture toughness under confining pressure. The pure mode-I fracture toughness (KIC) increased by a factor of about 3.7 under a σ3 of 28 MPa compared to that under ambient conditions. The variation of KIC was found to be linearly proportional to σ3. The pure mode-II fracture toughness (KIIC) increased by a factor of 2.4 upon increasing σ3 to 28 MPa. On the other hand, KIC at 116°C was only 25% more than that at ambient conditions. Some ductile behavior was displayed by the rock samples at a high temperature and confining pressure.

The combined effect of clay and moisture content on the behavior of remolded unsaturated soils

Abstract The behavior of unsaturated clayey soil is highly influenced by the coupled interaction between water and clay content. Various aspects of the behavior of artificial clay–sand mixtures with variable water content were experimentally studied. Laboratory tests were utilized for the determination of consistency limits, the stress–strain relationship, strength parameters, hydraulic conductivity, and volume change characteristics for various combinations of water and clay content in soil mixtures. Results presented for various clay–sand mixtures include: new normalized consistency limits; the combined effect of clay content and water content on the stress–strain relationship and on the strength parameters ( c and φ ); and the effect of clay content on hydraulic conductivity and swelling potential. The cohesion of clayey sand is found to increase with increasing water content to a certain limit, above which it decreases. The angle of internal friction for clayey sand is found generally to decrease with increasing water content. The degree of saturation is found to be better than the water content in explaining the strength behavior. The hydraulic conductivity sharply decreases with increasing clay content up to 40% beyond which the reduction becomes less significant. Simple empirical equations are proposed for predicting the swelling potential of clayey soils as a function of either the clay content or plasticity index.

Expansive characteristics of gypsiferous/anhydritic soil formations

Abstract Geology and climatic and environmental conditions have led to the formation of expansive soils in the Eastern Province of Saudi Arabia. Calcium sulphate, which commonly occurs in such soils, is well known for phase transformation and dissolution. Phase changes from gypsum to anhydrite and vice versa, and dissolution of these phases, add to the potential hazards of local expansive soils. This paper discusses the behaviour of the expansive soil formations of eastern Saudi Arabia containing gypsum and anhydrite.

EFFECTS OF CALCIUM SULFATE ON SWELLING POTENTIAL OF AN EXPANSIVE CLAY

Due to a reversible hydration-dehydration reaction, calcium sulfate undergoes phase transformations between a hydrated phase, gypsum, and a dehydrated phase, anhydrite. Due to the harsh climatic and environmental conditions in eastern Saudi Arabia, such phase changes add to the potential swelling hazards of local expansive clays. The adsorption of water by expansive soils and the hydration of anhydrite to gypsum create swelling pressure and are the sources of much damage to foundations throughout the world. This paper attempts to assess the swelling caused by the interaction of calcium sulfate phases, especially gypsum and anhydrite, with expansive clay. This assessment was primarily based on studying the geotechnical, mineralogical, and volume change characteristics of calcium sulfate-bearing soils. X-ray and thermal analyses were used to estimate the type and amount of minerals present during phase transformation of calcium sulfate. The swelling potential was determined using an improved version of the simple odometer and constant-volume tests. The conventional odometer is the device normally used in these tests. However, the size of soil samples, the complete confinement, and the rigidity of the conventional odometer impose serious limitations on the application of the laboratory results to actual field problems. Therefore, the authors investigated the use of a large-scale odometer with different mold sizes and shapes on the swelling potential of some mixtures of expansive clay and calcium sulfate phases. In addition, the soil fabric of these mixtures was investigated using scanning electron microscopy to explain the volume change behavior. The results of this investigation indicated that the swelling potential of clay-calcium sulfate mixtures decreased as the percentage of calcium sulfate was increased, and this reduction was more pronounced when gypsum was used. Swelling pressure was observed to be the highest in the conventional odometer and lowest in the large-scale square odometer mold.

A Plastic-damage Model for Stress-Strain Behavior of Soils

This paper presents a constitutive model for soil, which combines elements of plasticity with damage mechanics to simulate the stress-strain behavior. The model is primarily suitable for soil types that exhibit a postpeak strain-softening behavior, such as dense sand and stiff clay. The postpeak stress drop is captured by the elasto-damage formulation, while the plasticity is superimposed beyond the elastic range. The total strain increment is composed of an elasto-damage strain increment and a plastic strain increment. The elasto-damage strain increment is found using the elasto-damage formulation, while the plastic strain increment is found using either the Drucker-Prager classical plasticity model or as a function of damage strain. To implement this model, an experimental program was conducted on local cohesive and cohesionless soils. Various physical and mechanical properties of these soils were determined. Both triaxial tests and hydrostatic tests were performed under different confining pressures, in order to obtain the model parameters. These parameters were used to calibrate the model, which was coded in computer programs to simulate the stress-strain behavior of soils. The model was verified and found to be a good predictor of the geomaterial response for the selected stress path.

Stability of Vertically Bent Pipelines Buried in Sand

This paper discusses the stability of underground pipelines with preformed vertical bends buried in sandy soil. More specifically, the minimum cover height required to prevent the pipe from bowing under the action of forces due to temperature change and internal pressure is estimated. The variables considered include the pipe and soil materials, diameter, thickness, overburden height, bend radius, bend angle, internal pressure, fluid specific weight, and temperature variation. A comprehensive three-dimensional finite element analysis is carried out. The results are extracted from the output obtained. These results are put in a database which is used to develop general regression models to determine the relationships among the different variables. Different buckling modes are also considered. All of these results and models are entered into a computer software program for ready access.

Parameters for an Elasto-Plasto-Damage Model for the Stress-Strain Behavior of Dense Sand

This article presents a constitutive model for dense sand that exhibits a post-peak strain-softening behavior. This model combines elements of plasticity with damage mechanics to simulate the stress—strain behavior. The post-peak stress drop is captured by the elasto-damage formulation, while the plasticity is superimposed beyond the elastic range. The total strain increment is composed of an elasto-damage strain increment and a plastic strain increment. The elasto-damage strain increment is found using the elasto-damage formulation, while the plastic strain increment is found using either the Drucker—Prager classical plasticity model or as a function of the damage strain. To calibrate this model, an experimental program was conducted on dense sand at different relative densities. The various physical and mechanical properties of the sand were determined. Both triaxial compression tests and hydrostatic tests were performed under different confining pressures, in order to obtain the model parameters at various conditions. These parameters were used to calibrate the model, which was coded in FORTRAN computer programs to simulate the stress—strain behavior of dense sand. The model was verified and found to be a good predictor of the response of dense sand for the selected stress path.

Pullout Capacity of Block Anchor in Unsaturated Sand

Block anchor is an interface element used to restrain horizontal movement of structures. This paper investigates the effect of moisture conditions (or degree of saturation) on the pullout capacity of block anchor embedded in sand at three different moisture conditions. The approach taken consists of experimental work, and analytical calculation. The experimental work is pullout tests, made in the laboratory, on 0.15 x 0.15 x 0.15m concrete block anchors embedded in sand, at a depth of 0.15m. The sand is deposited in a 1.2 x 0.6 x 0.8m box using a pluviation method to ensure a uniform and reproducible density. Materials used were characterized to find their properties, and the equipments used were calibrated before usage. The load and the corresponding horizontal and vertical displacements were recorded. In addition, visual observations were made on the failed soil body. The experimental results are compared with the analytical calculations (by Rankine, Coulomb, and log spiral theories). The 3-D effect was also considered. The block anchor was found to have higher pullout capacity than a plate anchor. The results show that the moisture condition significantly affects the pullout capacity of the block anchor. The pullout capacity of the block anchor embedded in unsaturated (wet) sand is about double that for the block embedded in dry sand, while that for the block embedded in saturated sand is only about one half of that for the block embedded in dry sand. These findings have very significant implications in the analysis and design of the block anchor embedded in unsaturated sand. Also, these have contributions to the hazard risk assessment of block anchors embedded in sand subjected to variations in degree of saturation.

Utilisation of GIS Concepts for Foundation Design on Problematic Eastern Saudi Arabian Sabkha Soil

The design of foundations on a problematic soil is considered one of the most challenging issues for the geotechnical engineers. Sabkha, or salt flat, is considered as a problematic soil that exists in coastal areas. Ar-Rayyas sabkha is covering a wide area of the Eastern Province of the Kingdom of Saudi Arabia. King Fahd Suburb in the city of Dammam, Saudi Arabia, having been identified with an extensive presence of Ar-Rayyas sabkha, is considered for this study. Many structural failures of the residential buildings in the suburb were reported. The presence of sabkha is predominating the reasons of these failures. 59 investigatory boreholes were performed at the suburb and standard penetration N-values, have been obtained for the boreholes. A regression analysis was carried out, and a prediction model of the N-values using the coordinates and the depth intervals of the boreholes as regressors was provided. Due to the low accuracy of the model, an artificial neural network model was proposed to enhance the accuracy of the prediction; however, a further enhancement was required. Using GIS concepts, the suburb was mapped, and the borehole locations were projected with their attributes on the map. Different interpolation methods were utilised to predict and interpolate the N-values between the boreholes. The inverse distance weighting method yielded the most accurate model and therefore was selected to produce the final layouts for each depth interval. Finally, a framework for the design of foundations with the aid of GIS techniques was developed.

Cover Requirement and Stability of Horizontally Bent Buried Pipelines

The soil cover requirement for horizontally bent buried pipeline is discussed. The variables considered in this research include the pipe diameter and thickness, the radius and angle of the bend, the internal pressure, the fluid specific weight, the overburden height, the temperature rise, and the material used. A comprehensive threedimensional finite element analysis is run. The results obtained are utilized to develop regression models for the maximum allowed temperature change as well as the minimum overburden height. The relationships among the different variables are determined. To guard against elastic instability, several buckling mode are checked.